Add details to the 'nightly' page

[jypeter]

@doutriaux1 I've installed my first nightly version of CDAT, in order to get a working version of cdms2 on Win10+WSL (I can at least import cdms2 it, don't know yet if it actually works), but I have some burning questions that are not answered by the Obtain CDAT nightly packages

• how stable is the nightly version? do you guarantee at least that all the automated tests (or whatever your CI stuff does) work before tagging something as nightly?
• once I have installed a nightly CDAT, how do I update all or part of it (if I want to keep it up-to-date)? Is it better to update everything, or to update only some packages?
• what happens if install some stable external packages in my nightly version and update it? Will the external packages be updated as well (unless some dependency problems are reported)?
• what happens if I install a nightly version, clone it, install external packages in the cloned version and update the nightly version. Will the cloned version be updated as well?